Note: Descriptions are shown in the official language in which they were submitted.
~:27~ii22
412-1478
0114N
ELECTRICAL CIRCUITS AND COMPONENTS
BACKGROUND OF THE INVENTION
This invention relates to making electrical
components by the deposit and drying of fluids that
contain particles that have desired electrical and
mechanical properties.
In another aspect, the invention relates to
electroluminescent lamps, which typically are formed of
a phosphor-particle-containing layer disposed between
corresponding electrodes adapted to apply an excitation
potential to the phosphor particles, at least one of the
electrode layers being semi-transparent to light emitted
by the phosphors. The phosphor-containing layer is
provided with a barrier against moisture penetration to
prevent premature deterioration of the phosphors, and
permanent adherence between adjacent layers is sought to
avoid delamination, e.g. under constant flexing or
changes in temperature, particularly where the layers
are o materials having different physical properties as
this can also lead to premature ailure in prior art
electroluminescent lamps.
In the past, it has been recognized that
deposit of fluids, as by printing with polymeric inks
having electrical properties, would have a number of
advantages to the manufacture of electrical components,
including speed and accuracy of manufacture, low cost,
small product dimensions, etc. Limitations of known
inks and coating fluids as well as limitations in their
manner of use, however, have limited the applicability
of the techniques and the realizable electrical
performance characteristics In particular, high shear
stress mass transfer techniques, such as screen printing
:D~
~2~752;~
and doctor blade coating, have not found wide use for
products other than simple conductors.
There have been numerous and apparently
conflicting requirements for such techniques that have
stood in the way. Because nonuniformity of particle
distribution can result in non-uniform electrical
performance, there is a need for any such fluid
composition to hold the electrically active particles
in uniform suspension and inhibit their settling prior
to use and during the deposition and drying process.
The very high density of some electrically active
additives as compared to typical pigments, and their
general spherical shape, increases this demand.
It is important for any practical fluid
composition to have a high percentage of polymeric
binder, generally of the order of 50% percent, by
weight, in order to achieve a substantial dried coating
thickness in each application. Thickness is usually
needed to achieve the desired electrical properties as
well as mechanical strength and abrasion resistance.
There is further a need for such composition to
be highly thixotropic, i.e. have high "Ealse body", so
that while it is able to suspend the high density
additive particles, it yet can have temporary lower
viscosity under shear (i.e., be capable of "shear
thinning") to enable clean, accurate transfer of the
fluid composition to the substrate. Such accuracy of
formation is important because uniformity of thickness
determines uniformity of electrical properties.
There are further requirements that such
composition permit use of volatiles that have relatively
low evaporation rates at ambient temperatures in order
to achieve constant viscosity during an extended coating
or printing run during which the ink is exposed to the
~;~2752;~
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atmosphere. Changes in viscosity and concentration can
alter the characteristics of the deposit.
There are still further requirements that any
composition and its method of application be compatible
with substrates to which it is applied and to material
that may be subsequently applied to it so that no damage
is done to the various components of the circuit during
manufacture or use.
In the case of circuit components with
additives susceptible to deterioration in the presence
of moisture, such as phosphor particles for an
electroluminescent lamp, there are further stringent
requirements related to the protection of those
particles.
lS These and other requirements would present
themselves as obstacles to anyone who would seek to
broaden the use of fluid transfer techniques for the
formation of electrical components and cix~llits and to
lamps.
Summary of the Invention
According to the invent.ion it has been
discovered that a liquid dispersion of powder particles
comprised of polyvinylidene fluoride tPVDF)
simultaneously:
a) can suspend uniformly in desired
concentrations any of a wide variety of electrical
property additives, including crystalline, hard, dense
particles that are generally spherical in shape,
b) while containing a useful concentration of
such particles, can be deposited by high shear transfer
to a substrate in accurately controllable thickness and
contour,
c) when so deposited can be fused into a
continuous, uniform barrier film, the film itself having
low absorptivity, e.g., of moisture,
.;
so
-- 4 --
d) where desired, can, as one layer, be fused
with other such layers, containing other electrical
property additives, to form a monolithic electrical
component, and
(e) in general, can meet all requirements for
the making of many useful electrical circuit components,
including electroluminescent lamps, especially those
with additives harmed, e.g., by the presence of
moisture, by printing and coating with a high degree of
accuracy and controllability.
The discovery can be employed to form products
that are highly resistant to ambient heat and moisture
and other conditions of use. Despite markedly different
electrical properties between layers, the PVDF binding
polymer is found to be capable of a controllable degree
of interlayer penetration during fusing, which on the
one hand is s~,icient to provide monolithic properti~s~
enabling, e.g. repeated bending without delamination,
while on the other hand is sufficiently limited to avoid
adverse mixing effects between different electrical
additives in adjacent layers. PVDF can be employed as
the binder with additive particles having widely
different physical properties in adjacent layers, while
the overall multilayer deposit exhibits the same
coefficient of expansion, the same reaction to moisture,
and a common processing temperature throughout. Thus
each layer can be made under optimum conditions without
harm to other layers and the entire system will respond
uniformily to conditions of use.
Remarkable results have been obtained by the
simple techniques of silk screen printing and doctor
blade coating of successive layers. Of special
importance, it has been discovered that circuit
components that contain light-emitting phosphors and
~22~5%~
covering layers can be made which have unusual moisture
resistance, light emissivity and durability. The
moisture sensitivity of phosphors makes this a
particularly important discovery.
The invention accordingly features a method of
forming an electrical circuit component, and the
resulting product, especially electroluminescent lamps,
by depositing on a substrate, and drying, one or a
succession of superposed thin layers of a suspension of
polymer solid dispersed in a liquid phase, the
predominant constituent of the polymer being
polyvinylidene fluoride (PVDF), the liquid suspension
for at least one of the layers containing a uniform
dispersion of particles selected from the group
consisting of dielectric, resistive and conductive
substances of characteristic electrical values
substantially different from the respective values of
PVDF, the method including heating to fuse the polymer
continuously throughout the extent of the layer and
between layers, to form a monolithic unit.
While in certain cases homologs with
substantially similar properties may be employed, it IS
found that a polymer powder consisting essentially of
the homopolymer of PVDF produces electrical components
and layers of outstanding properties and, being also
commercially available, this polymer is presently
preferred.
In the preferred embodiment, each layer,
preceding the application of the next, is heated
sufficiently to fuse the polymer particles to form a
continuous film-like layer; the predominant constituent
of the liquid phase has substantially no solubility for
the polymer under the conditions of its deposit; the
liquid phase is predominantly formed from one or more
~2Z75~;~
-- 6
members selected from the group consisting of methyl
isobutyl ketone (MIBK), butyl acetate, cyclohexanone,
diacetone alcohol, diisobutyl ketone, butyrolactone,
tetraethyl urea, isophorone, triethyl phosphate,
S carbitol acetate, propylene carbonate, and dimethyl
phthalate; the liquid phase includes a minor amount of
active solvent selected to promote the stability of
suspension of the polymer particles in the liquid phase
without substantially dissolving the polymer; the liquid
phase includes a minor amount of one or more members
selected from the group consisting of acetone,
tetrahydrofuran (THF), methyl ethyl ketone (MEK),
dimethyl formamide (DMF), dimethyl acetamide (DMAC),
tetramethyl urea and trimethyl phosphate, in quantity to
lS promote the stability of the suspension of the polymer
particles in the liquid without substantially dissolving
the polymer; the liquid dispersion for a layer exhibits
a so tantial reduction in viscosity under high shear
stress and the layer is deposited by high shear
transfer; thy layer is deposited by silk screen
printing; the layer is deposited by blade coating; the
deposits are of predetermined, printed form; the
substrate and the deposit thereon comprise a flexible
unit; and the thickness of each of the layers is in the
range of between about .003 inch to .0001 inch.
Preferred Embodiment
We first briefly describe the drawings:
E'ig. 1 is a perspective view in section of an
electroluminescent lamp formed according to the
invention;
Fig. 2 is a side section view of the lamp taken
at the line 2-2 of Fig. l;
~2275Z~
-- 7 --
Fig. 3 is side section view of a portion of
side the lamp indicated in of Fig. 1, enlarged as viewed
through a microscope.
We first describe, in Examples A through D,
examples of selected electrical circuit components
formed as thin layers and then describe, in Example E, a
complete electrical circuit, in this case an
electroluminescent lamp, formed of a superposed series0 of the layers as described in Examples A through D.
Examples
EXAMPLE A - Dielectric Insulating Layer
To prepare the dielectric composition, 10 grams
of a PVDF dispersion of 45 percent, by weight,
polyvinylidene fluoride (PVDF) in a liquid phase
believed to be primarily carbitol acetate (diethyl
glycol monoethyl ether) were measured out. This
dispersion was obtained ommercially from Pennwalt
Corporation under the tradename "Kynar Type 202'1o As
the electrical property-imparting additive, 18.2 grams
of barium titanate particles tBT206 supplied by Fuji
Titanium, having a particle siæe of less than about 5
microns) were mixed into the PVDF dispersion. An
additional amount of carbitol acetate (4.65 grams) was
added to the composition to maintain the level of solids
and the viscosity of the composition at a proper level
to maintain uniform dispersion of the additive particles
while preserving the desired transfer perEormance. It
was observed after mixing that the composition was thick
and creamy and that the additive particles remained
generally uniformly suspended in the dispersion without
significant settling during the time required to prepare
the exampleD This is due, at least-in part, to the
number of solid PVDF particles (typically less than
about 5 microns in diameter) present in the composition.
1'r~ qe l a
~Z7~
A substrate was selected for its resistance to
the carrier fluid employed and for its ability to
withstand the extreme temperatures of treatment, e.g. up
to 500F/ as described below, in this case, a flexible
PVDF film. The composition was poured onto a 320 mesh
polyester screen positioned 0.145 inch above the
substrate Due to its high apparent viscosity, the
composition remained on the screen without leaking
through until the squeegee was passed over the screen
exerting shear stress on the fluid composition causing
it to shear-thin due to its thixotropic character and
pass through the screen to be printed, forming a thin
layer on the substrate below. The deposited layer was
subjected to drying for 2 1/2 minutes at 175F to
drive off a portion oE the liquid phase, and was then
subjected to heating to 500 F (above the initial
melting point of the PVDF) and was maintained at that
temperature for 45 seconds. This heating drove off
remaining liquid phase and alsc fui~ed the PVDF into a
continuous smooth film on the substrate.
The resulting thickness of the dried polymeric
layer was 0.35 mil (3.5 X 10 4 inch).
A second layer oE the composition as described
was screen~printed over the first layer on the
substrate. The substrate now coated with both layers
was again subjected to heating as above. This second
heating step caused the separately applied PVDF layers
to fuse together. The final product was a monolithic
dielectric unit having a thickness of 0.7 mil with no
apparent interface between the layers of polymer, nor
with the substrate, as determined by examination of a
cross-section under microscope. The particles of the J
additive were found to be uniformlyvdistributed
throughout the deposit.
The monolithic unit was determined to have a
dielectric constant of about 30.
~2~7~;~2
g
EXAMPLE B - Light Emitting Phosphor Layer
To prepare the composition, 18.2 grams of a
phosphor additive, zinc sulfide crystals (type #723 from
GTE Sylvania, smoothly rounded crystals having particle
size of about 15 to 35 microns) were introduced to 10
grams of the PVDF dispersion used in Example A. It was
again observed after mixing that despite the smooth
shape and relatively high density of the phosphor
crystals, the additive particles remained uniformly
suspended in the dispersion during the remainder of the
process without significant settling.
The composition was screen printed onto a
substrate, in this case a rigid sheet of polyepoxide,
standard printed circuit board material, through a 280
mesh polyester screen positioned 0.145 inch above the
substrate to form a thin layer. The deposited layer was
subjected to the two stage drying and fusing procedure
described in Example A to fuse the PVDF into a
continuous smooth film on the substra.e with the
phosphor crystals uniformly distributed throughout.
The resulting thickness of the dried polymeric
layer was 1.2 mils (1.2 X 10 3 inch).
The deposited film was tested UV and found to
be uniformly photoluminescent, without significant light
or dark spots.
EXAMPLE C - Semi-Transparent Conductive Front Lamp
Electrode
To prepare this conductive composition, 13.64
grams of indium oxide particles (from Indium Corporation
Of America, of 325 mesh particle size) were added to 10
grams of the PVDF dispersion used in Example A. An
additional amount of carbitol acetate (4.72 grams) was
added to lower the viscosity slightly to enhance the
transfer properties. It was again observed after mixing
that the additive particles remained uniformly suspended
~2~52~
-- 10 --
in the dispersion during the remainder of the process
without significant settling.
The composition was screen printed onto a
substrate, in this case a polyamide film, e.g., KAPTON~
supplied by E.I. duPont, through a 280 mesh polyester
screen positioned 0.5 inch above the substrate to form a
thin layer. The deposited layer was subjected to the
two stage drying and using procedure described in
Example A to fuse the P~DF into a continuous smooth film
on the substrate with the particles of indium oxide
uniformly distributed throughout.
The resulting thickness of the dried polymeric
layer was 0.5 mil (0.5 X 10 3 inch).
The deposited film was tested and found to have
conductivity of 10 ohm-cm, and to be light transmissive
to a substantial degree due to the light transmissivity
of the semi-conductor indium oxide particles and of the
matrix material.
E~MPLE D - Conductive Buss
To prepare this conductive composition, 15.76
grams of silver flake (from Metz Metallurgical
Corporation, of 325 mesh #7 particle size) were added to
10 grams of the PVDF dispersion used in the exarnples
above. The particles remained uniformly suspended in
the dispersion during the remainder of the process
without significant settling.
The composition was screen printed onto a
suitable substrate through a 320 rnesh polyester screen
positioned 0.15 inch above the substrate to form a thin
layer. The deposited layer was subjected to the two
stage drying and fusing procedure described in Example A
to fuse the PVDF into a continuous smooth film on the
substrate with the silver flake uniformly distributed
throughout.
a f
~75;~Z
-- 11 --
The resulting thickness of the dried polymeric
layer was 1.0 mil (1.0 X 10 3 inch).
The deposited film was tested and found to have
conductivity of lO 3 ohm-cm.
In the hollowing example we manufactured a
complete electroluminescent lamp lO, comprised of a
deposit of superposed thin polymeric layers as described
above having different characteristic electrical
properties, as described with reference to the
drawings.
EXAMPLE E
Referring to Fig. 1, the substrate 12 used in
this lamp configuration was flexible aluminum toil (4.2
mils) cut in pieces of size suitable for handling, e.g.
2 inches by 3 inches. The foil was cleaned with xylene
solvent.
A coating composition for forming dielectric
layeL 14 upon the substLate 12, in this case to act as
an insulator between the substrate/electrode 12 and the
overlying light-emitting phosphor layer 16 (described
below), was prepared as described in Example and
coated in two layers upon the substrate.
A coating composition for forming the light
emitting phosphor layer 16 was prepared as described in
Example B. The composition was superposed by screen
printing over the underlying insulator layer 14 and the
substrate with its coatings 14 and 16 was subjected to
the heating conditions described.
Subjecting the layers to temperatures above the
meIting temperature of the PVDF material caused the PVDF
to use throughout the newly applied layer and between
the layers to form a monolithic unit upon the substrate,
as shown enLarged as under a microscope in Fig. 3
227522
- 12 -
However, the interpenetration of the material of the
adjacent layers having different electrical properties
was limited by the process conditions to less than about
5 percent of the thickness of the thicker of the
5 adjacent layers, i.e. to less than about 0.06 mil so
that the different electrical property-imparting
additive particles remained stratified within the
monolithic unit as well as remaining uniformly
distributed throughout their respective layers.
The coating composition for forming the
semi-transparent top electrode 18 was prepared as
described in Example C. The composition was superposed
by screen printing upon the light-emitting phosphor
layer 16. The substrate with the multiple layers coated
15 thereupon was again heated to above the ~VDF melting
temperature to cause the semi-transparent upper
electrode layer to fuse throughout and to fuse with the
underlying light-emitting layer to form a monolithic
' unit. The indium oxide, though typically characterized
20 as a semiconductor, serves as a conductor here, and its
transparency enhances the light transmissivity of the
deposited layer.
The coating composition for forming the
conductive buss 20 was prepared as described in Example
25 D and was screen printed upon semi transparent upper,
electrode 18 as a thin narrow bar extending along one
edge of the electrode layer, for the purpose of
distributing current via relatively short paths to the
upper electrode.
This construction with connecting wires 34, 36
(Fig. 1) and a power source 38, forms a functional
electroluminescent lamp 10. Electricity is applied to
the lamp via the wires and is distributed~by the buss
layer 20 to the front electrode 18 to excite the
~2Z7S22
phosphor crystals in the underlying layer 16, which
causes them to emit light.
Due, however, to the damaging effect of, e.g~,
moisture on phosphor layer 16, it is desirable to add a
protective and insulative layer 22 about the exposed
surfaces of the layers of the lamp to seal to the
peripheral surface of the substrate 12. This layer 22
is also formed according to the invention, as follows.
The PVDF dispersion employed in Example A,
devoid of electrical-property additives, is screen
printed over the exposed surfaces of the lamp 10 through
a 180 mesh polyester screen. The lamp was dried for two
minutes at 175F and heated for 45 seconds at
500F. The coating and heating procedure was
performed twice to provide a total dried film thickness
of protective-insulative layer 22 of 1.0 mils. (By
using PVDF as the binder material in this and all the
underly ng layers, each layer has the same processing
requirements and restrictions. Thus the upper layers,
and the protective coating, may be fully treated without
damage to underlying layers, as might be the case if
other different binder systems were employed.)
The final heating step results in an
electroluminescent lamp 10 of cross-section as shown
magnified in Fig. 3. The polymeric material that was
superposed in layers upon flexible substrate 12 has
fused within the layers and between the layers to form a
monolithic unit about 3.4 mils thick that flexes with
the substrate. As all the layers are formed of the same
polymeric material, all the layers of the monolithic
unit have common thermal expansion characteristics,
hence temperature changes during testing did not cause
delamination. Also, due to the continuous film-like
nature of each layer due to the fusing of its
7522
14 -
constituent particles of PVDF and the interpenetration
of the polymeric material in adjacent layers, including
the protective layer 22 covering the top and exposed
side surfaces, the lamp was highly resistant to moisture
during high humidity testing, and the phosphor crystals
did not appear to deteriorate prematurely, as would
occur if moisture had penetrated to the crystals in the
phosphor layer.
In the following examples, the physical
properties of compositions useful according to the
invention, prior to the addition of additives, were
evaluated.
Viscosity
To determine the approximate range of viscosity
prior to addition of additives over which the
compositions af the invention are useful, two
compositions were prepared using isophorone as t
liquid phase and polyvinylidene fluoride (PVDF) powder
(461 powder, supplied by Pen~walt), which is
substantially insoluble in isophorone, i.e., it is
estimated that substantially less than about 5 percent
solvation occurs. The physical properties of the new
composition were adjusted by addition of PVDF powder or
isophorone until the first composition (Composition A)
had thickness or body at close to the lower end of the
range useful for screen printing, and the second
composition (Composition B) had body at close to the
high end of the useful range.
The weight proportions of the compositions and
the resultant viscosities are as shown in TABLE A.
TABLE A
Composition A Composition B
PVDF 65 83
Isophorone 56 58
Wt~ solids 53.4 5~.9
Viscosity 17,700 cps 200,000~ cps
~L~27~;2~:
- 15 -
The viscosity of the compositions was measured
using a Brookfield Viscosity Meter, Model LVF, at the #6
(low shear) setting. Composition A was tested using a
#3 spindle at a multiplication factor of 200X and gave
5 an average reading of 88.5. Composition B was tested
using a ~4 spindle at a multiplication factor of 2000X
and gave an average reading that appeared well in excess
of the maximum reading of 100.
The viscosity of the commercially available
10 Kynar 2~2 PVDF dispersion (Composition X) was tested on
the same equipment and registered a viscosity of
approximately 40,000 cps. (It is noted that while the
weight percentage of PVDF solids is lower in the
commercial product than in either of the test
15 compositions, a different solvent is employed in the
commercial system, so strict interpolation is not
possible.)
To demonstrate the shear thinning
characteristic of the composition, a standard coating
20 composition, in this case a dielectric composition
prepared as in example A, was subjected to further
testing. The viscosity of the coating composition was
tested in a Brookfield Viscosity Meter, Model LVF, as
described above, with a ~4 spindle operated at four
25 selected, different speed settings, the speed of the
spindle of course being directly proportional to the
shear between the spindle and the composition. As shown
in TABLE B, the viscosity of the composition decreased
dramatically with increased shear.
,,
gl Z~752~
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TABLE B
Brookfield Viscosity Meter, Model LVF
Spindle #4
Spindle Multiplier
5Setting Factor ReadingViscosity
6 1000 50 50,000 cps
12 500 64 32,000 cps
200 74 14,800 cps
100 86 8,600 cps
10 Solids Range
The weight percent solids of PVDF will vary
depending upon the nature of the carrier fluids
employed, and upon the physical properties of the
additive, e.g. upon particle surface area (particle
shape, spherical or otherwise, as well as particle size)
and particle density. The range of PVDE' solids present
in the overall coating composition can range between
about 50 percent, by weight, down to about 15 percent,
by weight. The preferred range is bet-~een about 25 and
45 percent, by weight.
_ther Embodiments
Numerous other embodiments are within the
following claims, as will be obvious to one skilled in
the art.
The protective layer 22 of the
electroluminescent lamp may be applied as preformed film
of polyvinylidene fluoride under pressure of 125 pounds
per square inch, and the lamp heated at 350 F for one
minute and then cooled while still under pressure. Each
separate layer applied may have a dry thickness of as
much as .010 inch, although thickness in the range
between about .003 inch to .0001 inch is typically
preferred. The protective layer may be applied as
preformed film of one or more other materials compatible
with the lamp structure, which alone or in combination
provide adequate protection against penetration of
~2275~2
- 17 -
substances detrimental to performance of the underlying
lamp.
As mentioned, the composition may be applied by
screen printing, or by various of the doctor blade
coating techniques, e.g. knife over roll or knife over
table. The shear-imparting conditions of screen
printing may also be varied, e.g. the squeegee may be
advanced along the screen at rates between about 2 and
200 inches per minute, and the size of the screen
orifices may range between about 1.4 and 7 mils on a
side.
Materials which consist essentially of
homopolymers of PVDF are preEerred. However, other
materials may be blended with PVDF, e.g. for improving
surface printability, for improving processability
during manufacturing, or for improving surface bonding.
An example of one material miscible in a blend with PVDF
is polymethyl methacrylate ~PMMA), e.g. employed at '
15 percent by weight of PVDF, preferably 5 to 10 percent
by weiyht. Also, other materials may be employed in
place of PVDF.
The guiding criteria or selection are low
moisture absorptivity, ability of particles to fuse at
elevated temperature to form a continuous moisture
barrier film, and, when applied to flexible substrate,
flexibility and strength. The general physical and
mechanical properties of PVDF (in homopolymer form)
appear in Table C.
TABLE C
General Physical and MechaniCal Properties
of Polyvinylidene Fluoride (PVDF)
Property ASTM Method Values
Specific Gravity D 792 1.75-1.78 g/ml
(109~3-111.3 lb/ft3)
122~S~:2
- 18 -
Property ASTM Method Values
Specific Volume D 792 0.56-0.57 ml/g
(15.5-15.8 in /lb)
Retractive Index D 542 1042 nO 25
Melting Point D 3418 156-168C
(312-334F)
Water Absorption D 570 0.04-0.06~
Tensile Strength @ D 638 25C 36-51 MPa
Yield 100C 19-23 MPa
(77F 5200-7400 psi
212F 2700-3400 psi)
Tensile Strength @ D 638 25C 36-52 MPa
Break 100C 19-23 MPa
(77F 5200-7500 psi
212F 2700-3400 psi)
Elongation @ Break D 638 25C (77F)
25-500%
100C (212F)
400-600%
Tensile Module D 638 1340-1515 MPa
(194-219 X 103 psi)
Stiffness in Flex D 747 1100-1730 MPa
(160-250 X 103 psi)
Flexural Strength D 790 59-75 MPa
(8.6-10.8 X 103 psi)
25 Flexural Modulus D 790 1200-1800 MPa
(175-260 X 103 psi)
Compressive Strength D 695 25C 55-69 MPa
~77F 8-10 X 103
p5i)
30 Izod Impact D 256 25C 160-530 kJ/m
(notched) (77 3.0-10.3
ft-lb/in.) -
Izod Impact D 256 77F 32 58
(unnotched)
ft-lb/in.)
Hardness, Shore D 2240 70-80
~2275~2
-- 19 --
Property ASTM Method Values
Hardness, Knoop Tukon 9 D 4-9.6
Coefficient of 0.14-0.17
Sliding Friction
5 -to Steel
Sand Abrasion D 968 4.01/um
(1021/0.001131
Tabor Abrasion Wheel 7.0-9.0 mg/1000 cycles
C5-17
1000
The liquid phase of the composition may be
selected from the group of materials categorized in the
literature as "latent solvents" for PVDF, i.e., those
with enough afEinity for PVDF to solvate the polymer at
15 elevated temperature, but in which at room temperature
PVDF is not substantially soluble, i.e., less than about
5 percent. These include: methyl isobutyl ketone
(MIBK), butyl acetate, ~clohexanone, diacetone alcohol,
diisobutyl ketone, butyrolactone, tetraethyl urea,
20 isophorone, triethyl phosphate, carbitol acetate,
propylene carbonate, and dimethyl phthalate.
Where additional solvation is desired, a
limited amount of "active" solvent which can, in greater
concentrations, dissolve PVDF at room temperature, e.g.,
25 acetone, tetrahydrofuran (THF), methyl ethyl ketone
(MEK), dimeth~l formamide (DMF), dimethyl acetamide
(DMAC), tetramethyl urea and trimethyl phosphate, may be
added to the carrier. Such limited amounts are believed
to act principally in the manner of a surfactant,
serving to link between the PVDF polymer particles and
the predominant liquid phase, thus to stabilize the PVDF
powder dispersion.
As will also be obvious to a person skilled in
the art, the viscosity and weight percent of PVDF solids
in the coating composition may also be adjusted, e.g. to
~L2~752~
- 20 -
provide the desired viscosity, suspendability and
transfer characteristic to allow the composition to be
useful with additive particles of widely different
physical and electrical characteristics.
The additives mentioned above are employed
merely by way of example, and it will be obvious to a
person skilled in the art that other additives alone or
in combination, or other proportions of the additives
mentioned may be employed according to the invention.
For example, for worming resistors, semiconductors and
conductors, suitable additives may be selected on the
basis of bulk resistivity or bulk density, or on the
basis of other criteria such as cost. The bulk
resistivities and bulk densities of examples of5 materials useful as additives are shown in TABLE D.
TABLE D
Material Resistivity Density
(ohm cm) (gm/cc)
Gold < 10-6 19.3
20 Silver < 10-6 10.5
Copper < 10-6 8.9
Brass < 10-6 8.5
Iron < 10-6 7.9
Tungsten < 10-5 19.4 ;
25 Nickel < 10-5 8.9
Cobalt < 10-5 8.6
Stainless Steel. < 10-5 8.0
Tin < 10-5 6.5
30 Indium Oxide 0.1 7.2
Zinc Oxide 1.0 5.6
Mica powder 106 __
Aluminum oxide > 106 4.0
Of course many other suitable materials are
available, e.g., alloys of the listed metals or others
may in some cases be employed in forming a conductor;
salts rendered stably semiconductive by the addition of
donor or acceptor dopants may in some case be employed
in forming a semiconductor; and glass (fiber, shot or
beads) or clay may in some cases be employed or
electrical resistance.
~Z~752~:
- 21 -
Similarly, additives useful as insulators or as
capacitors may be selected on the basis of dielectric
constant of the material as used in the composition or,
againl on the basis of density or other factors For
example, materials resulting in a composition having a
dielectric constant above 15 are useful for forming
capacitive dielectrics. Use of additives according to
the invention provides a composite layer with electrical
characteristics significantly different in degree from
that of PVDF above. Examples of materials with
sufficiently high dielectric constant are shown in TABLE
E for comparison with PVDF.
TABLE E
Dielectric
Constant
Material (approx.) Density (gm/cc)
Barium Titanate 10,000 6.0
Strontil~ Titanate 200 5.1
Titanium `_~G~:ide 100 3.8
PVDF 10 1.8
Additive particles suitable for use in
formation of an electroluminescent lamp include zinc
sulfide crystals with deliberately induced impurities
~"dopants"), e.g., of copper or magnesium.
Representative materials are sold by GTE, Chemical and
Metallurgical Division, Towanda, Pennsylvania, under
the trade designations type 723 green, type 727 green,
and type 813 blue-green.
What is claimed is:
. ,
